Organisms possess an internal ‘body clock’ that measures the passage of the day, although they respond directly to the influences of the sun. The site of this clock, its properties, and its molecular and genetic mechanisms are now beginning to be discovered. In humans, the clock enables the processes required for daytime activities and nocturnal sleep to be separated; but the system can go wrong or cause difficulties, as in the blind, anybody after a time-zone transition or during night work